You can set new widths using define (constants) or by feeding width/height parameters on the avatar function calls in the templates – The BP codex should have examples Only use CSS if you’re reducing the dimensions but only if slightly as you’re still calling a larger size image where it may not be needed.

@hugo
I’m sorry,i wrote it incorrectly. Avatar displays in resolution 100×100, but “buddypress” while addition changes its resolution in 35×35 in miniatures and because of that the picture displays very unintelligibly

That worked for me. Probably not the best route to altering it if you want to do a lot of CSS modifications to the “Group Forum Listing.” But if you want to just change the size to something larger this works.